In a telephone switching system using fiber optic links between various system elements, electro-optic transmitter and receiver modules are used to terminate each end of the optical link. Each serve as the electronic/optical interfaces between the equipment and the fiber. Electronic equipment must be protected from electro-static voltage (ESD), or transient voltages which it may be exposed to, by adjacent equipment, or craftsperson servicing the equipment. Such destructive voltages may be in the range of a few hundred volts to thousands of volts.
Because total protection from ESD is very difficult, separation of the electronic ground return path became a common practice. Any hits from ESD may then be conducted on metallic surfaces to the frame ground, which is in turn solidly connected to the regulatory building ground. Although optical signals are not affected by ESD, the electrical components can be.
Physically, the modules are located on a circuit card which is attached to one side of the backplane. In some instances, up to twelve modules including associated clock recovery circuits are located on such a card. Because of the small size of the card, which can be (10.5.times.3.5) inch, and the need to locate the modules close to the faceplate to make the optical receptacle accessible, small module size is required. In addition, the module should be able to relieve strain to the hybrid from forces associated with the fiber optic connector and cable, provide general mechanical protection, EMI shielding with grounding, heat transfer away from electronic components, and provide minimal resistance to the general flow of cooling air over the PCB.
Although electro-optic modules of small size already exist, these have an optical connector with a metallic liner or shell to provide increased rigidity. Even with the connector grounded to the equipment ground, it was found that unwanted signals and high voltage transients were affecting the operation of the internal components of the module, and other electronic circuits on the circuit card.
There is accordingly a need for an improved electro-optic module able to sustain high voltage transients, able to eliminate unwanted signals while retaining its small size and rigidity as well as meeting the above requirements.